Porous materials derived from block copolymers are crucial for numerous applications including size selective separations such as ultrafiltration for water treatment, the controlled delivery of drugs, the templating of surface patterns, and the production of functional inorganic nanostructures. The self-assembly of block copolymers into ordered morphologies provides a scalable approach for the synthesis of ordered materials that include uniform, nanoscale pores and high specific surface areas that may be chemically functionalized.
The selective etching of a sacrificial block of a block copolymer is the most widespread method of producing such porous polymers. Despite numerous etching chemistries reported in the literature, there remain challenges in balancing the extent of an etch process against the preservation of the nanostructure. For example, a widely used alkaline etch route has been noted to damage the quality of thin-film pattern transfer after only 15 minutes of mild 0.05 M NaOH contact, limiting thin film pattern retention to films of less than 50 nm. The etching of thick polymer films is highly challenging as a high selectivity of the etchant is crucial in order to to limit the degradation of the retained block, while the thickness of the polymer films calls for long exposure times in order to etch across multiple grain boundaries, which increases the likelihood of nanostructure degradation. For instance, etching through a poly(styrene-block-lactide) (PS-b-PLA) film of no more than about 1 millimeter (mm) in thickness with 0.5 M NaOH typically requires about 3-5 days, corresponding to an average etch rate of 1 to 1.5 nanometers per second (nm/s). In another example, complete PLA removal from PS-b-PLA films of about 0.32 mm thickness required shear alignment of the morphology to reduce the grain boundary concentration and a 44 hr etch with 0.5 M NaOH. Such methodology is highly problematic.
The preservation of the chemical and structural integrity of the retained matrix is crucial for nanostructure retention through a complete etch process. Evidence for nanostructure retention over macroscopic regions is generally provided by use of an ensemble measurement such as small-angle X-ray scattering (SAXS) that can sample a few mm3 to quantitatively confirm preserved lattice parameters over ˜1013 unit cells. NaOH-etched films have been known to exhibit a shift in the SAXS pattern. Such shifts of SAXS peak positions demonstrate a change to the scattering structure factor and are indicative of morphology shrinkage.
What are needed in the art are etchants and etching methodologies that can selectively remove the sacrificial block of polymeric thin films while retaining the nanostructure morphology of the film. Moreover, etchants that can etch thicker films (e.g., on the order of a few millimeters) quickly and with high selectivity for the sacrificial block and thereby successfully retain the porous nanostructure of the film would be beneficial in the art.